A digital communication system usually has one or more data buses for transporting information between various portions of the system. Each bus may be a two-wire balanced system or a single-wire with common ground unbalanced system and usually comprises one, four, eight, sixteen or thirty-two wires or paths. In addition, a bus may be unidirectional, in which case it is adapted to transfer information in only one direction or it may be bidirectional and is adapted for the transmission of data in both directions. Each path of the bus exhibits the characteristics of a transmission line including a characteristic impedance which may be predetermined by design choice of material, spacing of conductors, and other factors influencing the distributed elements of the transmission line.
It is important in a data bus system to have the minimum amount of noise generated by undesired reflectors at the driver and receiver gates. This result is achieved by matching the characteristic impedance of each transmission line with an impedance circuit connected to the bus.
The majority of transmission lines have fairly low characteristic impedances; these can range from about fifty to two hundred ohms with a nominal value of about one hundred and twenty ohms. The current necessary to obtain a reasonable voltage swing on such a bus line may be quite large. It is more difficult to couple noise into this low impedance, but it is more difficult to drive, and line drivers must be capable of supplying the necessary current.
The conventional bidirectional bus in a common ground unbalanced system usually comprises a transmission line, including a wire, having a driver gate and a receiver gate at each end of the wire. The gates employed as driver gates are off-the-shelf components available from the semiconductor manufacturers. They are usually three-state devices which when gated OFF offer an almost infinite impedance to the line connected at their output. Since a data bus is basically a transmission line exhibiting a characteristic impedance (Z.sub.0), a termination at the opposite end is necessary for each direction of driving. By conventional circuit theory (Thevenin's theorem), it may be determined that a matching impedance network for matching a Z.sub.0 of 120 ohms is a combination of two resistors from power rail to ground of about 220 and 330 ohms respectively with the junction of the resistors connected to the bus. This type of bidirectional bus arrangement is commonly used in present day digital equipment.
On the other hand, in an application such as an address bus for a memory system, a unidirectional bus system is used. In the realization of such a bus system, it is known to use a matching resistor at the driver end of the transmission line. This resistor is connected in series between a driver gate and the transmission line and is chosen to match its characteristic impedance. The advantage of the circuit is that if the input impedance of the receiver is high, very little power is dissipated and current only flow during the transition time which is twice the line delay time.